Coal is the most abundant natural energy source in the world. A significant portion of the United States domestic energy requirements are met by burning coal as a fossil fuel. There are various types of coal found within the United States., i.e., anthracite, semi-anthracite, low-volatile bituminous coal, medium and high volatile bituminous coal, sub-bituminous coal, and lignite. Coals such as anthracite and semi-anthracite typically have high ash and sulfur contents and therefore require beneficiation prior to use.
The primary purpose of coal beneficiation is to reduce the incombustible ash content, thus enhancing the heat content. Reduction in the ash content results in savings in transportation and ash disposal costs. Sulfur, mainly in the form of purite, is also reduced.
Another important economic factor to be considered in coal processing is the recovery and reuse of process water. Water is typically very expensive and there are often limits on total usage. Also, strict environmental controls prohibit or severely limit discharge of process water. Thus, it is imperative that solids be efficiently removed from the process water and thereafter that the water is recycled to the process stream.
Beneficiation of coal is effected using two primary properties of coal, i.e., (1) differences in specific gravity between coal and its impurities, and (2) differences in surface characteristics between coal and its impurities. Since the higher ash content fractions are usually found in the finer coal sizes, some plants only screen out these sizes to beneficiate the coal. However, since the quantity of such fine coal is on the rise, even this is treated.
A coal beneficiation plant may be broadly categorized into specific gravity separation and fine coal treatment. In gravity separation, cleaning units make use of the differences in specific gravity between coal and its impurities to effect separation. Normally, the specific gravity of the clean coal is less than its impurities. Some commonly used equipment for gravity separation are: jigs, heavy medium baths and cyclones, washing tables, water only cyclones and spirals.
Fine coal treatment incorporates a flotation cell(s), clean coal filter and thickener. In the flotation cell, a collector and frother are added to the flotation feed. The collector (e.g., diesel oil #2) selectively imparts hydrophobicity to the coal particles. This increased hydrophobicity makes the air bubbles more likely to attach to the coal particles. The frother (e.g., an alcohol based product) reduces the surface tension of the air/water interface, thus making a stable froth.
The concentrate (i.e., clean coal) from the flotation cells goes to the clean coal filter and is dewatered. The tailings from the flotation cell go to the thickener where they are thickened and discharged. After being treated in the thickener, the concentrated solids may be further treated or dewatered to produce a material suitable for disposal by landfill or other means.
In the thickener, the inorganic solids are typically treated with coagulants and flocculants to enhance settling. Typically, the coagulants and flocculants are added at several points along the feed line to the thickener and in different sequences. Coagulation is the destabilization by surface charge neutralization of stable negatively charged particles that are in suspension (i.e., settleable or dispersed) through the utilization of inorganic salts or cationic polyelectrolytes. Flocculation is the aggregation of finely divided particles which are suspended in a liquid through the utilization of an entrapping agent (i.e., an inorganic flocculant) or a bonding agent (i.e., an organic flocculant) that brings the particles together.
Although some inorganics, principally alum and iron salts, are still used as coagulants, water soluble organic polymers are more commonly used. These water-soluble polymers can be either naturally occurring or synthetic polymers. The principal natural polymers are starch and guar, both of which are high-molecular weight polymers of simple sugars (i.e., polysaccharides). Specifically, starch is a polymer of glucose consisting of a mixture of linear (amylose) and branched (amylopectin) segments.
An advantage of synthetic polymers is that they can be tailored to a specific application. Therefore, a wide range of commercially available coagulants and flocculants of varying charges, compositions and molecular weights have been developed. The most widely used synthetic coagulants are polydiallyldimethyl ammonium chloride as described in U.S. Pat. No. 2,926,161 and condensation polymers of dimethyl amine and epichlorohydrin such as those described in U.S. Reissue Pat. Nos. 28,807 and 28,808. These polymers vary greatly in molecular weight, typically ranging from several thousand to as high as 500,000. Condensation polymers are made in solution form, and are available commercially as aqueous solutions containing a relatively low weight percent polymer. Polydiallyldimethyl ammonium chloride is a vinyl addition polymer, which (at the molecular weights used for coagulation) has also been made in solution form. Typical commercially available polydiallyldimethyl ammonium chloride is available in aqueous solutions containing 1-20% by weight polymer. Copolymers of diallyldimethylammonium chloride and acrylamide having utility for the dewatering of mineral slurries have been described in U.S. Pat. No. 4,673,511.
Dry water soluble polymers such as dry polydiallyldimethyl ammonium chloride have also been used to dewater coal refuse slurries. These polymers have met with some success, dissolving in the refuse thickener over a period of 45 to 60 seconds. Such time is long enough to provide continuous feed of fresh polymer into the coal/clay slurry. Disadvantages of dry polymer however are that it produces dust; if not carefully fed, may produce gelled agglomerates which can foul feeding equipment; and is difficult to handle because the material is packaged in bags which must be moved into proximity of the thickener, thereby preventing feeding from a remote point more convenient to chemical delivery and handling. The polymers of the present invention overcome these deficiencies by providing dissolution of the polymer over a time period which insures a continuous supply of fresh coagulant to the thickener.
Polymeric materials similar to those of the instant invention have been disclosed in several patents. U.S. Pat. No. 4,950,725 issued to Flesher et at. describes a water-soluble polymeric material with a triallylamine quaternary ammonium salt cross-linker. U.S. Pat. No. 5,210,298 issued to Shimomura et at. describes an acrylate-containing polymer with a triallylamine cross-linker, and U.S. Pat. No. RE 32,649 issued to Brandt et at. describes a hydrogel forming, water-insoluble polymer slightly cross-linked with triallylamine. EP 161-763A describes a water-soluble and swellable co-polymer formed of a diallyl ammonium salt, monofunctional vinyl compound and polyfunctional vinyl compound. However, the inventors have discovered that the unique properties of this polymer make it ideal for use as a controlled-release coagulant for concentrating aqueous suspensions of inorganic solids.
The present invention is directed to the use of water-in-oil emulsions of certain medium molecular weight polydiallyldimethyl ammonium chloride polymers which may be used as coagulants in the thickening process during coal beneficiation. These water-in-oil emulsions of diallyldimethyl ammonium chloride polymers exhibit improved performance or activity in coal tailings thickening over solution polymers or conventional water-in-oil emulsions of diallyldimethyl ammonium chloride polymers. In addition, due to the slow inversion rate of the water-in-oil emulsion of the diallyldimethyl ammonium chloride polymer prior to addition to the thickener feed, the performance of the polymer is improved. As such, the polymer coagulants of this invention exhibit higher performance than lower molecular weight polymers of diallyldimethyl ammonium chloride, either in solution or water-in-oil emulsion form. In addition, the polymers of the subject invention eliminate the handling difficulties associated with dry coagulants.